Optical Waveguide Having Bistable Transmission States Suitable for Optical Logic Circuits

ABSTRACT

An optical circuit comprises a bistable optical waveguide ( 34 ) having a first and a second transmission state. The waveguide is more transmissive to light of a given wavelength in the second state than in the first state. A first light source ( 11 ) and a second light source ( 21 ) emit light of a first and second wavelength respectively and are coupled to the waveguide at one end. Selective transmission of a sufficient amount of light of the first wavelength through the waveguide “sets” the waveguide, causing it to switch from the first into the second state, whereas transmission of a sufficient amount of light of the second wavelength “resets” the waveguide causing it to switch back from the second into the first state. A sensing or reading (“test”) light source ( 36 ) is arranged at the other end of the waveguide to transmit a sensing light signal through the waveguide ( 34 ) in the opposite propagation direction to that of light of the first and second wavelengths. This sensing light source can be an external light source or an “internal” source provided by spontaneous emission in the waveguide. A sensor ( 38 ) is arranged to detect the amount of the sensing light signal transmitted through the waveguide ( 34 ). In this way the waveguide can be set into a given transmission state, which can be determined at a later time by measuring the amount of the sensing light signal transmitted. The optical circuit therefore exhibits a memory effect and may be used to produce an all-optical bistable logic circuit such as an optical latch or an optical flip-flip. Typically, the waveguide ( 34 ) is a doped optical fibre, such as an Erbium-Ytterbium (Er—Yb) doped fibre. Light of the first (set)/second (reset) wavelengths excites or de-excites respectively the dopant ions in the fibre thus tuning its transmission.

FIELD OF THE INVENTION

The present invention relates to optical circuits, typically multi- orbi-stable and to a method of operating such a circuit, in general, andin particular (but not exclusively) to the provision of a bistableoptical circuit and an optical memory.

BACKGROUND OF THE INVENTION

In future photonic or optical networks including technologies such asoptical packet switching, all-optical bistable devices (commonly knownas “latches” or “flip flops”) will be key elements to provide for packetbuffering, self-routing and bit-length conversion. Few devices andarchitectures for all-optical bistable devices have been demonstrated,mainly based on the bistable operation of laser diodes and semiconductoroptical amplifiers. In particular, some kinds of bistable laser diodes,such as vertical-cavity surface-emitting lasers (see H. Kawaguchi etal., “Pitchfork bifurcation polarization bistability in vertical-cavitysurface-emitting lasers”, Electron. Lett., vol. 31, no. 2, pp. 109-111,1995) and coupled laser diodes (Y. Liu et al., “Three-state all-opticalmemory based on coupled ring lasers” IEEE Photon. Technol. Lett., vol.15, no. 10, pp. 1461-1463, October 2003) have been investigated for therealisation of all-optical bistable devices. Furthermore, with thedevelopment of optical networks, it is desirable to develop all-opticalmemories.

By “light”, “optical” and cognate terms herein, we not only refer tovisible light, but also Infra Red (IR) and Ultra Violet (UV) light as iscommon in the art.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is providedan optical circuit, comprising:

an optical waveguide having a first and a second state, the opticalwaveguide being more transmissive to light of a given wavelength in thesecond state than in the first state;

a first light source coupled to the optical waveguide, so as toselectively transmit light of a first wavelength through the opticalwaveguide; and

a second light source coupled to the optical waveguide, so as toselectively transmit light of a second wavelength through the opticalwaveguide, in which transmission of the first wavelength light throughthe optical waveguide causes the optical waveguide to be in the secondstate, and transmission of the second wavelength light causes theoptical waveguide to be in the first state;

the optical circuit further comprising a test light source, arranged totransmit a test light signal of the given wavelength through the opticalwaveguide, and an output for the test light signal after it has passedthrough the optical waveguide, in which the test signal is coupled tothe optical waveguide such that, in use, the test optical signalcounter-propagates to at least one of the first and second opticalsignals.

Accordingly, it is possible to set the optical waveguide into a givenstate and then determine by means of the test signal, typically at somelater time, what state the optical waveguide is in. The optical circuitmay therefore “remember” which state it is in so as to provide a novelmultistable or bistable optical circuit—which could be considered an“optical latch” or “optical flip-flip”. Furthermore, it may be possibleto use such a circuit as an optical memory. The counter-propagation ofthe test signal is a simple and reliable way to be able to separate thetest signal from the light emitted by the first and second sources.

When acting as a latch or flip-flop, transmission of the light from thefirst light source may act as a “set” signal, whereas transmission ofthe light from the second light source may act as a “reset” signal. Whenacting as a memory, transmission of the light from the first lightsource may act as a “write” signal, whereas transmission of the lightfrom the second light source may act as a “clear” signal. In each case,the logic high or “1” state may be considered to be the more transparentstate of the optical waveguide (where the light from the test lightsource can successfully traverse the optical waveguide) whereas thelogic low or “0” state may be considered to be the less transparentstate of the waveguide (where the light from the test light source wouldnot be able to as successfully traverse the optical waveguide).

Accordingly, the level at which the test light signal is output may beused as the output of the circuit. The output may comprise a sensor forsensing the level of the test light signal after it has passed throughthe optical waveguide. Alternatively, it may simply provide an opticalcoupling for coupling the test light signal to at least one furtheroptical circuit, hence possibly providing an all-optical circuit.

The optical waveguide typically comprises an optical fibre, although itis also envisaged that it could comprise a waveguide integrated onto aMMIC or other integrated circuit. In the preferred embodiment, theoptical waveguide comprises a doped transmission medium typically dopedwith Erbium (Er) and optionally Ytterbium (Yb). Indeed, the opticalwaveguide may comprise or consist of an Er—Yb doped optical fibre.

Using an Er—Yb doped transmission medium makes use of the absorption andstimulated emission characteristics of such a medium. By appropriatechoice of the energy levels of the ions in the medium (typically, bychoosing the amounts and ratios of the Erbium and Ytterbium dopants), itis possible to define the first and second states as defined above. Thefirst wavelength can then be chosen such it is close to the maximumabsorption of the medium (typically due to Erbium), typically so as toincrease the energy of ions in the medium and to place the medium into amore transmissive state, whereas the second wavelength can be chosen tobe one where stimulated emission dominates in the medium, typicallydecreasing the energy of ions in the medium and placing the medium in amore absorptive state. The first wavelength may be substantially 1535nm, typically 1535.8 nm, and the second substantially 1565 nm, orsubstantially 1568.0 nm.

In order to change the optical waveguide from first to second states orvice versa, the first and/or second signals may only need to be pulses;the first and second optical sources may therefore be arranged totransmit such pulses. The pulses may have a pulse width of the order of10 ns; preferably the pulses are of sufficient strength and duration tocause the majority of ions in the waveguide to switch between energylevels corresponding to the two states. The fibre will most preferablyhold whichever state it was in last for a period after the applicationof a pulse; this period may be at least a microsecond, possibly at least10 or even 20, 30, 40 or 50 microseconds. This is most desirable for useas an optical latch or flip-flop.

The test signal can be, and advantageously is, very much smaller interms of transmitted power, than the light emitted by the first andsecond light sources. The power of the test signal may be at most 10%,1%, 0.1% or 0.01% of the peak power transmitted by the first or secondlight sources. Keeping the test signal at a low power is firstlyefficient use of power and secondly avoids inadvertently changing thestate of the optical waveguide. Preferentially, the test signal is of adifferent wavelength to the first and/or second wavelengths, and may besubstantially 1541 nm, or 1550.9 nm.

In one embodiment, the test light source comprises spontaneous emissionfrom the transmission medium. Alternatively, the test light source maybe a light source distinct from the transmission medium but coupledthereto. It may comprise a laser.

The optical waveguide preferably has a first and a second end. The firstand second optical sources may be coupled to the first end of theoptical waveguide, whereas the test optical source may be coupled to thesecond end of the optical waveguide.

The circuit may further comprise an optical circulator coupled to thefirst end of the optical waveguide. This circulator may have first,second and third ports, the first port being coupled to the first andsecond light sources, the second to first end of the optical waveguideand the third to the sensor. As is common with circulators, thecirculator may be arranged such that a signal incident on the first portis transmitted on the second port and a signal incident on the secondport is transmitted on the third port. Typically, a signal incident onthe third port may be transmitted on the first port, but that is not ofgreat importance for the present invention.

Accordingly, the light emitted by first and second light sources can betransmitted down the optical waveguide from the first end towards thesecond end, whilst the test signal can be transmitted from the secondend to the first end and then directed towards the sensor. Given thatthe test signal may be very much smaller than the light emitted by thefirst or second light sources, this is a simple and reliable way toseparate the test signal from the light emitted by the first and secondsources.

Furthermore, the optical circuit may be further provided with anisolator at the second end of the optical waveguide, arranged to discard(typically by absorption) light transmitted from first to second endsbut to allow transmission of light into the second end for transmissionvia the optical waveguide to the first end. Accordingly, the isolatormay allow the test light signal to be introduced at the second end, butto discard the light transmitted by first or second signals after it hastraveled along the optical waveguide. This protects the test lightsource from damage.

The first and second light sources may be coupled together by means of acoupler. The coupler typically combines the light transmitted by bothsources and transmits that to the optical waveguide, possibly by meansof the circulator, if such is provided. Alternatively, a further couplermay be provided, arranged such that, in use, signals received from theoptical waveguide are passed to the output at a first port of thefurther coupler, but signals incident on a second port of the furthercoupler are passed to the optical waveguide with minimal cross-talk tothe output. The first port may be connected to the output, whereas thesecond port may be connected to the coupler. A uni-directional isolatormay be provided between the second port and the coupler, to only allowsignals to pass in the direction from the coupler to the furthercoupler.

According to a second aspect of the present invention there is provideda method of operating an optical circuit, the optical circuit comprisingan optical waveguide having a first and a second state, the opticalwaveguide being more transmissive to light of a given wavelength in thesecond state than in the first state, the method comprising:

in order to switch the optical waveguide from the first state to thesecond state, transmitting light of a first wavelength through thewaveguide;

in order to switch the optical waveguide from the second state to thefirst state, transmitting light of a second wavelength through thewaveguide;

in order to determine which of the first or second states the waveguideis in, transmitting a test signal through the waveguide and determininghow much of the test signal is transmitted through the opticalwaveguide.

The step of determining how much of the test signal is transmitted maycomprise the step of measuring the amplitude or power of the test signalonce it has traversed the optical waveguide. The determination of howmuch of the test signal is transmitted may be a simple relative test; a“low” transmitted amplitude or power may signify that the waveguide isin the first state, whereas a “high” transmitted amplitude may signifythat the waveguide is in the second state.

Typically, the optical waveguide comprises an optical fibre, although itis also envisaged that it could comprise a waveguide integrated onto aMMIC or other integrated circuit. In the preferred embodiment, theoptical waveguide comprises a doped transmission medium typically dopedwith Erbium (Er) and optionally Ytterbium (Yb). Indeed, the opticalwaveguide may comprise or consist of an Er—Yb doped optical fibre.

The steps of changing the optical waveguide from first to second statesor vice versa may comprise transmitting a pulse of the first or secondwavelength as appropriate. The pulse may have a pulse width of the orderof 10 ns; preferably the pulse is of sufficient strength and duration tocause the majority of ions in the waveguide to switch between energylevels corresponding to the two states. The fibre will most preferablyhold whichever state it was in last for a period after the applicationof a pulse; this period may be at least a microsecond, possibly at least10 or even 20, 30, 40 or 50 microseconds. This is most desirable for useas an optical latch or flip-flop.

The step of transmitting the test signal advantageously comprisestransmitting a signal which is very much smaller in terms of transmittedpower than the light emitted at first and second wavelengths whenchanging the state of the waveguide. The power of the test signal may beat most 10%, 1%, 0.1% or 0.01% of the peak power at first or secondwavelength. Keeping the test signal at a low power is firstly efficientuse of power and secondly avoids inadvertently changing the state of theoptical waveguide. Preferentially, the test signal is of a differentwavelength to the first and/or second wavelengths, and may besubstantially 1541 nm.

The method may comprise use of an optical circuit according to the firstaspect of the invention. The method may make use of any of the optionalfeatures described with respect to that first aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood and appreciated more fully fromthe following detailed description taken in conjunction with thedrawings in which:

FIG. 1 is a diagram illustrating an optical circuit according to oneembodiment of the present invention;

FIG. 2 is a graph showing the spectra of the “set” and “reset” pulsesused in the optical circuit of FIG. 1;

FIG. 3 is a graph showing the spectra of the test signal used in theoptical circuit of FIG. 1;

FIG. 4 a shows the timing of the application of the pulses of FIG. 2 tothe optical circuit of FIG. 1;

FIG. 4 b shows the resultant measured test signal after it has beentransmitted through the optical fibre of FIG. 1;

FIG. 5 shows a diagram of an all-optical memory realized in accordancewith a second embodiment of this invention and inserted in a testcircuit,

FIG. 6 shows graphs of contrast ratios in optical means as a function ofthe time elapsed after a writing, and

FIG. 7 shows temporal diagrams of reading writing, erasing and outputsignals in the memory in accordance with this invention.

DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

An optical circuit forming a first embodiment of the invention is shownin FIG. 1. A first signal continuous wave signal generator 11 having afirst wavelength λ_(S) of 1535 nm is used to generate the first signal.This signal can be considered to be the flip-flop “set” signal. Thefirst signal is pulsed using a programmable electric wave generator 14and a Mach Zender Modulator (MZM) 12. The first signal is amplified bymeans of an Erbium Doped Fibre Amplifier (EDFA) 13 up to 23 dBm (200mW). The modulation efficiency of the MZM 12 is polarization dependentso a Polarization Controller (PC) 15 is inserted into the signal pathjust before the MZM 12.

In the same way, the second signal, forming the reset for a flip-flopsignal, is generated through a tunable laser 21 having second wavelengthλ_(R) of 1565 nm. The signal thus generated is modulated by a second MZM22 under the control of a second electric wave generator 24 andamplified by means of a second EDFA 23 up to 23 dBm (200 mW). A secondPC 25 is inserted into the signal path just before the second MZM 22.The two electronic wave generators 14, 24 are linked by asynchronisation generator 40 so that the timings of “set” and “reset”pulses can be controlled relative to one another.

The first and second signals are then coupled together by means of 50:50coupler 31. This outputs at two separate ports a 50:50 mixture of thefirst and second signals; an oscilloscope 32 can be attached to one portfor diagnostic purposes. The other port is coupled to a circulator 33having three ports. The first and second signals are provided at thefirst port of the circulator, and are transmitted by the circulator toits second port.

The second port of the circulator, and hence the first and secondsignals are coupled to into a first end of 1.5 m of Erbium-Ytterbiumdoped fibre 34, forming the optical waveguide of the present invention.The fibre is arranged such that, for a given wavelength, the firstsignal (at the first wavelength) will cause the fibre 34 to becomerelatively transparent—the second state of the fibre—whereas the secondsignal (at the second wavelength) will cause the fibre 34 to becomerelatively opaque—the first state of the fibre.

The second end of the fibre 34 is terminated by an isolator 35. Thisallows signals to travel only into, and not out of, the fibre 34 at thesecond end. Accordingly, the first and second signals will be absorbedby the isolator 35 after they have traversed the fibre 35.

Attached to the isolator 35 is a test signal source 36 comprising afurther tunable laser. This is generated as a continuous wave at a testsignal wavelength of λ_(P)=1541 nm even though it is, in principlepossible to transmit this signal across the entire bandwidth of thefibre 34. The test signal wavelength corresponds to the given wavelengthdiscussed above.

The isolator 35 therefore allows the introduction of the test signalinto the fibre whilst absorbing the first and second signals. The testsignal will be launched into the fibre 34 in a counter-propagating senserelative to the first and second signals. Once the test signal hastraversed the fibre, it reaches the circulator 33. The circulatortransmits the test signal to its third port, where it is amplified by alow noise preamplifier 37, and for visualisation purposes photodetectedby a photodetector 38 to convert it into an electrical signal, which isdisplayed on an oscilloscope 39.

To decrease noise, the circuit can be provided with band-pass filters torestrict the wavelengths travelling down a certain branch to only thosethat should be passing down that branch. In the circuit shown in FIG. 1,filters 51, 52, and 53 are provided; filter 51 ensures that the firstsignal only comprises wavelengths around the first wavelength 1535 nm,filter 52 similarly ensures that the second signal comprises onlywavelengths around the second wavelength 1565 nm and filter 53 ensuresthat only signals having a wavelength around that of the test signal(1541 nm) pass to be detected by the photodetector 38.

FIG. 2 shows the spectra of the first or “set” signal (left-hand peak)and of the second or “reset” signal (right-hand peak). FIG. 3 shows thespectrum of the test signal before it is introduced into the fibre 34.

The operation of the optical circuit can be demonstrated with FIGS. 4 aand 4 b. These Figures schematise a periodic flip-flop operation showingin FIG. 4 a the relative timings of input set (first signal) and reset(second signal) pulses and in FIG. 4 b the output test signal asvisualised at oscilloscope 39. The repetition frequency of set and resetpulses trains is approximately 900 KHz both with a pulsewidth of 10 ns.Considering the power of set and reset signals at the fibre 34 input(port 2 of the circulator 33), we obtain a peak power of greater than 30dBm (1 W) for both set and reset pulses, thus result in a set pulseenergy E_(S)=18.48 nJ and reset pulse energy E_(R)=19.82 nJ.

When a “set” signal is transmitted, a signal of the first wavelength istransmitted through the fibre 34. This causes ions in the fibre 34 tobecome excited and cause the fibre to become more transparent to lightof the wavelength of the test signal—the second state of the fibre 34.This can be seen in FIGS. 4 a and 4 b, where a “set” pulse causes theoutput test signal to increase in amplitude. Applying a “reset” signalof the second wavelength to the fibre 34 causes the ions to lose energyand descend into the lower of the two states. Accordingly, thetransparency of the fibre 34 decreases at the wavelength of the testsignal (the fibre enters the first state) and so the amplitude of theoutput test signal decreases. Accordingly, the system has two stablestates and will remember which of “set” and “reset” occurred last. Thecircuit therefore operates as an all optical latch or “flip-flop”.

In the example shown, the second states was maintained for a period of422 ns, but this level can be considered constant even for dozens ofmicroseconds because of slow decay of excited ions through spontaneousemission. Both transition times are approximately 10 ns, because bothup- and down-conversion of ions completely exploits the energy of setand reset pulse energy. By increasing the power of set and reset pulsesand decreasing their pulsewidth, it is possible to speed up rising andfalling edge times. High-level memorization times of up to dozens ofmicroseconds have been measured.

FIG. 4 b has been obtained with a reading input power of −12 dBm (62.5μW): increasing the reading power up to 0 dBm (1 mW) it is possible toobtain lower Optical Signal to Noise Ratio (OSNR) and an higher valuefor the difference in measured output test signal amplitudes between thestates. In such a case the amplitude of the lower state increases,maintaining almost constant the contrast ratio between the two statesand therefore the possibility to distinguish high and low energy levels.

With reference to FIGS. 5 to 7, FIG. 5 shows diagrammatically a memoryin accordance with a second embodiment of this invention designated as awhole by reference number 110 and inserted in a test circuit.

In accordance with the principles of this invention, the memory 110utilizes the property of an energy-storing erbium-doped optical fibre111. This can be considered as a form of optical waveguide. In theexperimental realization, the use of 10 m of fibre was foundadvantageous.

The operating principle is as follows.

Consider a two-level energy system for the erbium-doped fibre 111. If anoptical signal with appropriate energy spreads through the means, it isabsorbed with a probability that depends on the number of ionspopulating the energy levels and on the absorption cross-section of thefibre. When the signal is absorbed, the ions are transferred to theexcited level. At the same time, a spontaneous and stimulated emissioncan occur with a probability closely linked to the input energyintensity, to the emission cross sections, and to the number of excitedions. By increasing the input energy, the process reaches a condition ofequilibrium and the input energy is completely transferred to theoutput.

In this manner, it is possible to define two work conditions, to wit,absorption and transparency. In particular, the binary number 1 can beassociated with the state of transparency of the erbium-doped fibre andthe binary number 0 with the absorption state of the same means withoutexcitement.

The memory state can be verified by injecting a reading signal or alow-power test signal into the fibre 111. If it reaches the output port,the stored bit is 1, or if it is completely absorbed, the stored bit is0. The writing of a 1 can be obtained using a high-energy opticalimpulse at a wavelength near maximum absorption of the erbium. Theexcited state of the erbium decays in a few milliseconds, showing abehaviour similar to the discharge of a condenser in an electric DynamicRandom Access Memory (DRAM). If the memory cell needs to be erased, asuitable erasing impulse at a wavelength where the stimulated emissiondominates takes the excited erbium ions down to the minimum energystate. The advantages of this architecture are a long storing time (upto the order of the millisecond) and the possibility of reading the cellat any time (rather than at fixed instants as happens with Fibre DelayLines). Moreover, structures based on Er—Yb waveguides suited to beingintegrated can be devised, reducing the imprint and the cost of theoptical memory.

The contrast ration (CR) of the memory is defined as follows:

${{C\; {R\lbrack{dB}\rbrack}} = {10\; {\log_{10}\left( \frac{V_{1}}{V_{0}} \right)}}},$

where V₁ is the voltage referred to the reading signal received when abit 1 is stored, while V₀ is the voltage caused by the not idealabsorption of the reading signal when a bit 0 is stored. CR representsdirectly the capability of discriminating two bits; high CR values aredesirable during all the reading time. Because of the lifetime of theerbium ions, the value read associated with a 1 decreases exponentiallyin time. It is possible to give the performance of the memory cell in CRterms at the highest value for the signal read immediately after thewriting of a 0.

CR is measured as a function of the temporal position of the readingsignal with regard to the impulse that writes a 1. The broken line inFIG. 6 shows a decrease in the CR in time from 12.3 dB to 4.2 dB. Thesevalues can be not high enough to allow recognizing two different bits.In addition, it takes a long time to erase the memory cell if one waitsonly for spontaneous emission. For these reasons, an erasing signalcapable of stimulating emission is used so as to reduce the excited ionsresponsible for the fibre transparency. By using the erasing signal, thestraight line of the contrast ratio shifts upward (solid line of FIG. 6)so that in the storing time the relationship is always kept above 13 dB(in this case it varies between 13 dB and 21 dB).

A device in accordance with this invention comprises a memory cell 110shown by way of example in FIG. 5 and comprising in turn an input 112for the a first optical “writing” signal and an input 113 for thereading or “test” optical signal. The memory state is detectable at anoptical output 114.

The reading signal is advantageously launched through the fibre 111counter-propagating as regards the writing signal. In this manner, anoptical filter is not necessary and the proposed solution is thussimpler and more economical.

There is also advantageously an input 115 for a second “erasing” opticalsignal. In the realization shown in FIG. 5 the erasing signal was chosenco-propagating with the writing signal.

To insert the writing and erasing signals and to extract the outputsignal, two known couplers 116, 117 arranged in cascade at one end ofthe fibre 111 were used. The first is connected to the writing anderasing input and the second receives the signal output from the firstto send it to the fibre and extract the output signal. Advantageously,to prevent that from the inputs might come out return signals that couldcause unsteadiness in the lasers and optical amplifiers connectedthereto, two known optical insulators 118, 119 were used with onearranged in series with the reading input 113 (with propagationdirection from the input to the optical fibre) and the other arrangedbetween the output of the input coupler 116 and the output coupler 117and propagation direction from the coupler 116 to the coupler 117.

For the operational tests, the memory 110 is connected to two opticalsources 120, 121 of impulses having power and duration sufficiently highto emit impulses of suitable energy to produce the writing and erasingeffects in the doped optical means 111 and to a source 122 of impulseshaving power and duration sufficiently low to produce the readingimpulses without alteration of the memory state.

In the example, to obtain the reading and writing signals, twocontinuous wave laser sources 123, 124 are amplified by respectiveerbium Doped Fibre Amplifiers (EDFA) 125, 126 to then be modulated. Inparticular, for the writing signal an λ_(w)=1535.8 nm continuous wavelaser 123 optically amplified by means of the EDFA amplifier 125 up to27 dBm is used. Modulation of the writing signal is obtained by using aprogrammable wave generator 127 and an acoustic optical modulator (AOM1)128.

In the same manner, the erasing signal is generated with theλ_(e)=1568.0 nm laser 24 amplified up to 25 dBm by the EDFA 126 andmodulated using a programmable wave generator 129 and an acousticoptical modulator (AOM2) 130.

The reading signal is also generated by a continuous laser source 131modulated by means of a modulator 132 controlled by a programmable wavegenerator 133.

In this experiment the wavelength of the reading signal is λ_(R)=1550.9nm even though, as a principle, it can be tuned over the entire band ofthe erbium fibre. Lastly, the reading signal at the output 114 isphoto-detected and viewed through an oscilloscope 134. It was foundadvantageous that the test signal impulse has energy between 600 nJ and800 nJ.

FIG. 7 diagrams a two-bit storing operation showing the writing signal(a), the reading signal (b) without erasing, the erasing signal (c), andthe reading signal (d) when erasing is used.

As may be seen in FIG. 7, at first a binary FIG. 1 is written using arectangular impulse with temporal duration of T_(w)=1.5 ms (FIG. 7 a).

It was found useful that the writing impulse have more that 600 mJ ofenergy and preferably around 752 μJ. In FIG. 7 the impulse has impulseenergy of E_(W)=752 μJ. The cell keeps the bit for a time not longerthan 2.5 ms when the second bit must be written or the same bitrefreshed. In FIG. 7, a 0 is written (that is, no new writing impulse isinserted) after 2.5 ms from the writing of a 1.

The value of the bit is read using a rectangular impulse of 50 μs. FIG.7( b) shows the impulse detected in output to the memory without the usethen of an erasing signal. A certain pedestal can be observed under theimpulse because of the spontaneous emission that falls within the outputfilter band. This pedestal decreases slowly and lowers the CR.

The erasing impulse was found useful between 300 μJ and 600 μJ andpreferably around 316 μJ. FIG. 7( c) shows an erasing signal withtemporal duration of approximately 1 ms and associated energy E_(E)=316μJ.

The signal read in this case is traced in FIG. 7( d) where the pedestalwas emphasized to throw light on the erasing signal effect. As seen, theerasing impulse strongly depopulates the second energy level whileconsiderably increasing the CR, as shown in FIG. 6, so that, with theerasing signal, the CR is held between 13 dB and 21 dB.

Thanks to the use of an optical memory cell based on an optical means(in particular, a silica fibre) doped with erbium, an efficientcompletely optical memory is obtained representing another promisingbuilding block for climbing over the gap between all-optical andelectrical packet switching. Thanks to the introduction of an erasingsignal, erasing of the memory was speeded up and the contrast ratio (CR)was considerably increased so as to have a clear and welldistinguishable distinction between bit 1 and bit 0. In the experimentaldevice, it was possible to measure refresh times of approximately 2.5 msand CR between 21 dB and 13 dB. This opens new frontiers towards storingof the light in erbium-doped waveguides.

It can be seen that optical circuits according to the present inventionare advantageous even if the medium is so much efficient (both Er andEr/Yb doped fibre are not optimised for such applications). Betterresults would be obtained using a medium whose absorption cross-sectionsare negligible with respect to emission cross sections in the resetregion (or in a spectral region where the reset signal can beallocated).

1-31. (canceled)
 32. An optical circuit, comprising: an opticalwaveguide having a first state and a second state, the optical waveguidebeing more transmissive to light of a given wavelength in the secondstate than in the first state; a first light source coupled to theoptical waveguide, and configured to selectively transmit a firstoptical signal comprising light of a first wavelength through theoptical waveguide to place the optical waveguide in the second state; asecond light source coupled to the optical waveguide, and configured toselectively transmit a second optical signal comprising light of asecond wavelength through the optical waveguide to place the opticalwaveguide in the first state; a test light source configured to inject atest light signal of the given wavelength into the optical waveguidesuch that the test light signal counter-propagates through the opticalwaveguide relative to at least one of the first and second opticalsignals; and an output configured to output the test light signal afterit has traversed the optical waveguide.
 33. The circuit of claim 32wherein the output comprises a sensor configured to sense a level of thetest light signal after it has traversed the optical waveguide.
 34. Thecircuit of claim 32 wherein the output comprises an optical couplerconfigured to couple the test light source to at least one furtheroptical circuit.
 35. The circuit of claim 32 wherein the opticalwaveguide comprises an optical fiber.
 36. The circuit of claim 32wherein the optical waveguide comprises a waveguide integrated onto anintegrated circuit.
 37. The circuit of claim 32 wherein the opticalwaveguide comprises a doped transmission medium.
 38. The circuit ofclaim 37 wherein the transmission medium is doped with Erbium (Er). 39.The circuit of claim 37 wherein the transmission medium is doped withYtterbium (Yb).
 40. The circuit of claim 32 wherein the first and secondoptical sources are configured to generate pulses of light.
 41. Thecircuit of claim 40 wherein the pulses have a pulse width of about 10ns.
 42. The circuit of claim 40 wherein the optical waveguide isconfigured to remain in its previous state for a period of time afterthe generation of a pulse.
 43. The circuit of claim 42 wherein theperiod of time is between about one microsecond and about 50microseconds.
 44. The circuit of claim 32 wherein the test light sourcetransmits the test light signal at a power that is lower than the powersat which the first and second light sources transmits their respectivefirst and second optical signals.
 45. The circuit of claim 32 whereinthe test optical signal has a wavelength that is different than at leastone of the first and second wavelegnths.
 46. The circuit of claim 32wherein the test light source comprises a spontaneous emission from thetransmission medium.
 47. The circuit of claim 32 wherein the test lightsource comprises a light source that is distinct from the transmissionmedium, but is coupled to the transmission medium.
 48. The circuit ofclaim 47 in which the test light source comprises a laser.
 49. Thecircuit of claim 32 wherein the optical waveguide has a first end and asecond end, and wherein the first and second light sources are coupledto the first end of the optical waveguide, and the test optical sourceis coupled to the second end of the optical waveguide.
 50. The circuitof claim 49 further comprising an optical circulator coupled to thefirst end of the optical waveguide, the optical circulator comprising:first, second and third ports; the first port being coupled to the firstand second light sources; the second port being coupled to the first endof the optical waveguide; and the third port being coupled to theoptical circuit output.
 51. The circuit of claim 50 further comprisingan isolator disposed at the second end of the optical waveguide, andconfigured to: discard light transmitted from the first end to thesecond end; and allow transmission of light into the second end fortransmission via the optical waveguide to the first end, so as to allowthe test light signal to be introduced at the second end, and discardthe light transmitted by first or second optical sources after the lighthas traveled along the optical waveguide.
 52. A method of operating anoptical circuit, the optical circuit including an optical waveguidehaving a first and a second state, and configured to be moretransmissive to light of a given wavelength in the second state than inthe first state, the method comprising: transmitting light of a firstwavelength through the waveguide to switch the optical waveguide fromthe first state to the second state; transmitting light of a secondwavelength through the waveguide to switch the optical waveguide fromthe second state to the first state; transmitting a test signal throughthe waveguide and determining how much of the test signal is transmittedthrough the optical waveguide to determine whether the optical waveguideis in the first state or the second state.
 53. The method of claim 52wherein determining how much of the test signal is transmitted throughthe waveguide comprises measuring at least one of an amplitude and apower of the test signal once it has traversed the optical waveguide.54. The method of claim 52 wherein transmitting light of a firstwavelength to switch the optical waveguide into second state comprisestransmitting a pulse of light of the first wavelength, and whereintransmitting light of a second wavelength to switch the opticalwaveguide into first state comprises transmitting a pulse of light ofthe second wavelength.
 55. The method of claim 54 wherein the pulseshave a width of about 10 ns.
 56. The method of claim 54 wherein thepulses of light are of sufficient strength and duration to cause amajority of ions in the optical waveguide to switch between energylevels that correspond to the two states.
 57. The method of claim 54wherein the optical waveguide maintains a previous state for a period oftime after the application of a pulse.
 58. The method of claim 57wherein the period of time is between about a microsecond and about 50microseconds.
 59. The method of claim 52 wherein transmitting the testsignal comprises transmitting the test signal at a power that is lowerthan a power at which the light is emitted at first and secondwavelengths when switching the state of the waveguide.
 60. The method ofclaim 59 wherein the test signal transmit power is between about 0.01%and about 10% of the peak power at first or second wavelength.
 61. Themethod of claim 52 wherein the test light source comprises spontaneousemission from the transmission medium.
 62. The method of claim 52wherein the test light source comprises a light source distinct from thetransmission medium but coupled thereto.